Perpendicular magnetic recording medium and magnetic storage apparatus

ABSTRACT

An improved perpendicular magnetic recording medium and an improved magnetic storage apparatus are provided, which are suitable for high speed and high density magnetic recording. A magnetic under layer of a two-layered perpendicular magnetic recording medium includes three layers: a ferromagnetic layer; a non-magnetic layer; and a ferromagnetic layer, wherein the ferromagnetic layers are antiferromagnetically coupled with each other, thereby preventing a magnetic flux from a magnetic wall in a magnetic under layer from entering a read back head.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a perpendicular magnetic recordingmedium suitable for high density magnetic recording, and a magneticstorage apparatus using the same.

2. Description of the Related Art

Along with the rapid propagation of personal computers, work stations,and the like, there has been a growing demand for increasing thecapacity of a magnetic disk apparatus, which is the core of anon-volatile file system. In order to increase the capacity of amagnetic disk apparatus, it is necessary to increase the recording bitdensity, i.e., the plane recording density. A recording system employedfor currently-used magnetic disk apparatuses is called “in-planerecording system”. In this system, a ferromagnetic layer having a largecoercivity in a direction parallel to the disk substrate plane is usedas a recording medium, and information is recorded by magnetizing therecording medium in a direction in the substrate plane. In such a case,bit “1” corresponds to a magnetic inversion section where two adjacentportions are oppositely magnetized to each other, i.e., at an angle of180°. In order to increase the in-plane recording density, it isnecessary to increase both the bit density in the circumferentialdirection of the disk (linear recording density) and the bit density inthe radial direction (track density). While the track density is limitedby the track width formation process or the positioning precision of theread/recording head, they are primarily only technical problems. It isbelieved that the linear recording density, on the other hand, issubject to a limitation in principle due to the characteristics of therecording medium.

In an in-plane recording system, two oppositely-magnetized portionsexist facing each other with a magnetic inversion point therebetween,thereby creating a large demagnetizing field. Due to the demagnetizingfield, a transitional region of a finite width is formed in the magneticinversion section. The width of the magnetic inversion region needs tobe at least smaller than the bit interval. Accordingly, in order toincrease the linear recording density, it is necessary that the mediumis magnetized despite the demagnetizing field. More specifically, it isnecessary to reduce the thickness of the recording magnetic layer whileimproving the coercivity of the medium. Thus, the linear recordingdensity is greatly limited by the magnetic properties of the medium. Ina standard in-plane magnetic recording system, it is desirable that theratio of the linear recording density with respect to the track densityis about 10. In order to realize a recording density of 50 Gb/in² undersuch a condition, the bit interval in the circumferential direction isabout 25 nm. A magnetic properties estimation with a simple model showsthat a medium in which the width of the magnetic inversion region is 25nm or less needs to have a thickness of 15 nm or less and a coercivityof 5 kOe or more.

However, with a coercivity over 5 kOe, it is difficult to ensure arecording magnetic field with which the medium can be sufficientlymagnetized. Moreover, with a Co-alloy-based magnetic layer, when thethickness of the magnetic layer is 15 nm or less, the substantial volumeof the medium crystal grain is so small that the magnitude of thethermal energy of the grain is non-negligible with respect to themagnetic anisotropy energy thereof. As a result, the influence of thethermal fluctuation becomes significant, thereby posing a problem ofthermal decay, where the magnitude of the recording magnetizationdecreases over time. When one attempts to ensure the crystal grainvolume with the crystal size in the in-plane direction, there will be anincrease in the medium noise, whereby a sufficient S/N ratio cannot beobtained. Thus, difficulties in principle are expected in realizing anin plane recording density of 50 Gb/in² or more while achieving asufficient thermal decay resistance and low noise.

A perpendicular magnetic recording system is a system in which athin-film medium is magnetized in a direction perpendicular to the filmplane thereof. It is believed that a perpendicular magnetic recordingmedium is different from an in-plane magnetic recording medium in theprior art in terms of the recording principle and the mechanism in whichmedium noise occurs. In the perpendicular magnetic recording system,adjacent magnetized portions are not facing each other but are in anantiparallel arrangement, whereby there is no influence of ademagnetizing field. Therefore, it would be expected that a magneticinversion can be achieved within a very narrow region, and it is easierto increase the linear recording density. Moreover, since the demand forreducing the thickness of the medium is not as strong as that for anin-plane recording medium, it is possible to ensure a high resistance tothe thermal decay. Thus, the perpendicular magnetic recording system hasbeen attracting public attention as a system that is essentiallysuitable for high density magnetic recording, and various mediummaterials and structures have been proposed therefor. Variousperpendicular magnetic recording systems include those in which a singleperpendicular magnetic layer is used and those in which a magnetic underlayer is provided on a perpendicular magnetic layer. A technique using atwo-layered perpendicular magnetic recording medium having a magneticunder layer is described in, for example, IEEE Transaction on Magnetics,Vol. MAG-20, No. 5, September 1984, pp. 657-662, “Perpendicular MagneticRecording-Evolution and Future”. A medium in which a perpendicularmagnetic layer made of a CoCr alloy is provided on an under layer madeof a soft magnetic material such as a permalloy is a candidate for theperpendicular magnetic recording medium for this system.

In order to realize a magnetic storage apparatus capable of high densitymagnetic recording of 50 Gb/in² or more based on a perpendicularmagnetic recording system using a two-layered perpendicular magneticrecording medium, it is necessary to reduce the medium noise. The mediumnoise occurs from both the perpendicular magnetic layer and the magneticunder layer, and particularly the spike-shaped noise occurring from themagnetic under layer has been a problem. An example of such noise isdescribed in, for example, IEEE Transaction on Magnetics, Vol. MAG-20,No. 5, September 1984, pp. 663-668, “Crucial Points in PerpendicularRecording”. In order to address such a problem, a method for forming anin-plane magnetic layer under a magnetic under layer has been proposedin the art, as seen in Journal of the Magnetics Society of Japan, Vol.21, Supplement No. S1, pp. 104-108 “Increasing S/N of Three-LayeredPerpendicular Medium and Stability of Recorded Signal”. However, it wasnot sufficient for realizing a magnetic storage apparatus capable ofhigh density magnetic recording of 50 Gb/in² or more.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a perpendicularmagnetic recording medium having low noise characteristics for realizinga high recording density of 50 Gb/in² or more, thereby facilitating therealization of a high density magnetic storage apparatus.

In order to realize a perpendicular magnetic recording medium having lownoise characteristics, the present invention provides a perpendicularmagnetic recording medium including a non-magnetic substrate, a magneticunder layer, a perpendicular magnetic layer and a protective lubricantlayer, the perpendicular magnetic layer and the protective lubricantlayer being provided on the non-magnetic substrate via the magneticunder layer, wherein the magnetic under layer includes at least one setof a layered structure of ferromagnetic layer/non-magneticlayer/ferromagnetic layer, and the magnetizations of the ferromagneticlayers are coupled with each other in an antiparallel state. The noisedue to the magnetic under layer can be reduced by preventing a leakageflux occurring from a magnetic wall in the magnetic under layer fromentering the read back head while securing the magnetic wall present inthe magnetic under layer so that it is not easily moved.

Specifically, the present invention provides a perpendicular magneticrecording medium, including a non-magnetic substrate, a magnetic underlayer, and a perpendicular magnetic layer, the perpendicular magneticlayer being provided on the non-magnetic substrate via the magneticunder layer, wherein the magnetic under layer includes at least onenon-magnetic layer sandwiched by adjacent ferromagnetic layers.

The non-magnetic layer of the magnetic under layer may be a non-magneticmetal layer made of a material selected from the group consisting of Ru,Rh, Ir, Cr and an alloy thereof, and each of the ferromagnetic layersadjacent to the non-magnetic layer of the magnetic under layer may be amagnetic layer made of a material selected from the group consisting ofCo, Ni, Fe and an alloy thereof. Magnetizations of the two ferromagneticlayers adjacent to the non-magnetic layer of the magnetic under layerare coupled with each other in an antiparallel state.

The present invention also provides a magnetic storage apparatus,including a perpendicular magnetic recording medium, driving means fordriving the perpendicular magnetic recording medium, a magnetic headincluding a write section and a read back section, means for relativelymoving the magnetic head with respect to a magnetic recording medium,and signal processing means for inputting a signal to the magnetic headand reading back an output signal from the magnetic head, wherein themagnetic recording medium is the above-mentioned perpendicular magneticrecording medium.

According to the present invention, since the noise due to the magneticunder layer of the perpendicular magnetic recording medium issignificantly reduced, the S/N ratio is significantly improved, and itis possible to realize a magnetic recording apparatus providing a highrecording density.

The nature, principle, and utility of the invention will become moreapparent from the following detailed description when read inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic cross-sectional view illustrating a structure ofan exemplary perpendicular magnetic recording medium according to thepresent invention;

FIG. 2A and FIG. 2B illustrate exemplary magnetic properties of amagnetic under layer;

FIG. 3A and FIG. 3B illustrate an Ru layer thickness dependency of themagnetic properties of the magnetic under layer;

FIG. 4 is a schematic cross-sectional view illustrating a single poletype recording head and a magnetic recording medium;

FIG. 5A is a cross-sectional view illustrating a read back head and amagnetic recording medium according to a conventional example, and FIG.5B illustrates the waveform of a read signal;

FIG. 6A is a cross-sectional view illustrating a read back head and amagnetic recording medium according to the present invention, and FIG.6B illustrates an exemplary waveform of a read signal;

FIG. 7 is a schematic cross-sectional view illustrating an exemplaryperpendicular magnetic recording medium according to the presentinvention;

FIG. 8 is a schematic cross-sectional view illustrating a single poletype recording head, a GMR read back head and a magnetic recordingmedium;

FIG. 9 is a schematic diagram illustrating a structure of a magneticstorage apparatus; and

FIG. 10 is a schematic cross-sectional view illustrating anotherexemplary perpendicular magnetic recording medium according to thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Embodiments of the present invention will now be described withreference to the drawings.

EXAMPLE 1

Using a glass substrate having a diameter of 2.5 inches, a magneticrecording medium having a cross-sectional structure as illustrated inFIG. 1 was produced by a DC magnetron sputtering method. A magneticunder layer including a first layer 42, a second layer 43 and a thirdlayer 44 was formed on a substrate 41. A perpendicular magnetic layer(CoCrPt) 52 having a thickness of 25 nm and a protective layer 53 havinga thickness of 5 nm were formed in this order on the magnetic underlayer via an intermediate under layer (TiCr) 51 having a thickness of 5nm. Two different media were produced, one in which the first layer 42and the third layer 44 were both made of Co with a thickness of 30 nm,and the other in which the first layer 42 and the third layer 44 wereboth made of CoFe with a thickness of 30 nm. In both media, the secondlayer 43 was made of Ru.

FIG. 2A to FIG. 3B illustrate the results of measurements performed forthe Ru layer thickness dependency of the magnetic properties of thethree-layered magnetic under layer in order to optimize the thickness ofthe Ru layer. Note that the thickness of the Co layer was set to be 10nm for the first layer 42 and the third layer 44. Substantially the sameresults were obtained when CoFe was used for the magnetic layers.

FIG. 2A and FIG. 2B illustrate magnetization curves for two differentCo/Ru/Co three-layered magnetic under layers in which the thickness ofthe Ru layer was 1.4 nm and 0.7 nm, respectively. First, for the Rulayer having a thickness of 1.4 nm, it can be seen that the layerexhibits a behavior that is similar to that of a single ferromagneticlayer, thus indicating that the two Co layers are coupled together in aparallel state. On the other hand, for the Ru layer having a thicknessof 0.7 nm, it can be seen that the magnetization M at magnetic field=0is substantially zero, thus indicating that the two Co layers arecoupled together in an antiparallel state, and a substantial couplingmagnetic field of about 2.1 kOe has occurred from the magnetic field atwhich the magnetization is saturated. As described above, the Co/Ru/Cothree-layered structure may be in either a parallel state or anantiparallel state depending upon the thickness of the Ru layer. Inorder to realize an intended magnetic under layer, the Co layers need tobe coupled in an antiparallel state. In view of this, the magneticproperties were examined while varying the thickness of the Ru layer.The results are shown in FIG. 3A and FIG. 3B.

FIG. 3A illustrates the Ru layer thickness dependency of the magneticfield Hs at which the magnetization is saturated. It can be seen thatthe maximum point and the minimum point of Hs alternately appear as thethickness of the Ru layer is increased. On the other hand, in the Rulayer thickness dependency of the residual magnetization Mr illustratedin FIG. 3B, Mr takes the maximum value when Hs takes the minimum value,and vice versa. The former of the two regions of the thickness of the Rulayer corresponds to FIG. 2A, and the latter corresponds to FIG. 2B.

In view of the results shown above, the thickness of the Ru layer wasset to be 0.7 nm, which is within the antiparallel coupling region, inthis example. Since the thickness of each Co layer was 30 nm in thisexample, the antiparallel coupling magnetic field (equivalent to Hs) wasabout 500 Oe.

In order to evaluate the characteristics of the recording media asdescribed above, signals were recorded thereon by using a single poletype recording head. FIG. 4 is a schematic cross-sectional viewillustrating the recording head and the recording medium used in theexperiment. Note that the details of the medium are not shown in thefigure. The recording head includes a main pole 61, a secondary pole 62and an excitation coil 63. When a current is allowed to flow through thecoil 63, the main pole 61 is excited, and a recording magnetic flux 65from the tip of the main pole 61 passes through a magnetic under layer64 into the secondary pole 62. The magnetic flux is confined to a smallcross-sectional area at the tip of the main pole 61. As a result, astrong magnetic field is applied through a perpendicular magnetic layer66, thereby producing a magnetization pattern 67 corresponding to thesignal. In the experiment, two different signals of 60 kFCI and 600 kFCIwere recorded. For the purpose of comparison, the same signals wererecorded also on a conventional recording medium whose magnetic underlayer is a single Co layer having a thickness of 60 nm. An observationof the surface of the media having signals recorded thereon with an MFMconfirmed that sharp magnetic inversion had been recorded within anarrow track on both media.

Next, the recorded signals were read back with a read back head forcomparison in the S/N ratio and the resolution. FIG. 5A is across-sectional view illustrating the read back head used in theexperiment and the conventional recording medium having signals recordedthereon, and FIG. 5B illustrates an exemplary read signal. A shield typeGMR head was used for the read back operation. The shield type GMR headincludes a GMR element 73 being sandwiched between shields 71 and 72,which are arranged with an interval of 80 nm therebetween. The intervalbetween the recording medium surface (the surface of a perpendicularmagnetic layer 74) and the surface of the read back head is about 30 nm.

Generally, an ideal waveform of a read signal from a perpendicularrecording medium having a magnetic under layer is rectangular. However,referring to FIG. 5B, a read signal waveform 78 includes spike-shapednoise 79 superimposed thereon. This is likely to be the result ofmultiple magnetic domains being formed in a magnetic under layer 76irrespective of a recording pattern 75, as illustrated in FIG. 5A,whereby a static magnetic field is produced from a magnetic wall 77, towhich the read back head responds.

FIG. 6A and FIG. 6B illustrate exemplary evaluation results for themedium of the present invention. As can be seen from FIG. 6B, a readsignal waveform 78 takes a generally ideal rectangular shape and doesnot include spike-shaped noise as in the convention example. This islikely to be the result of two Co layers 76 and 80 of the magnetic underlayer being magnetically coupled in an antiparallel state, asillustrated in FIG. 6A, thus creating magnetic domains of oppositepolarity in the upper and lower Co layers, respectively, whereby astatic magnetic field from the magnetic wall 77 and that from a magneticwall 81 are canceled out by each other. Thus, with the recording mediumof the present invention, there can be expected an effect of reducingthe noise from the magnetic under layer.

Next, the two media as described above were compared with each other interms of the macroscopic characteristics of the signals. Table 1 belowshows a comparison between the two media in terms of the S/N ratio andthe resolution where a signal of 600 kFCI was magnetically recorded. TheS/N ratio was evaluated as the ratio of the half value of the readoutput at a low recording density with respect to the noise at a highrecording density (600 FCI). The resolution was measured as theproportion (percentage) of the read output for a signal recorded at 300MHz (600 kFCI) with respect to the read output for a signal recorded at30 MHz (60 kFCI). TABLE 1 Present invention Prior art S/N(dB) 25.5 24.3Resolution(%) 12.2 9.6

It has been shown that the magnetic recording medium of this exampleprovides an improved S/N ratio and thus is a desirable high densitymagnetic recording medium. It has also been shown that the magneticrecording medium of this example is suitable for high speed read/writeoperations since there is only little deterioration in read signals whenthe signals are recorded at a high frequency. The magnetic recordingmedium produced in this example was evaluated by using a GMR head as aread back element. As a result, it was possible to realize an on-trackbit error rate of 10⁻⁸ at an in-plane recording density of 55 Gb/in²,thereby confirming that the magnetic recording medium of this examplehad a sufficient performance to be used as a high density magneticstorage apparatus.

It has been shown that the magnetic recording medium of this example isa desirable high density magnetic recording medium with the S/N ratioand the high frequency write characteristics thereof which aresignificantly improved as compared with those of the comparativeexample. The magnetic recording medium produced in this example wasevaluated by using, as a read back element, a high efficiency read backhead based on the magnetic tunneling phenomenon. As a result, it waspossible to realize an on-track bit error rate of 10⁻⁶ at an in-planerecording density of 80 Gb/in², thereby confirming that the magneticrecording medium of this example had a sufficient performance to be usedas a super high density storage apparatus.

EXAMPLE 2

A magnetic storage apparatus as illustrated in FIG. 9 was produced byusing a disk (FIG. 7) obtained by depositing the perpendicular magneticrecording medium of Example 1 on both sides of a substrate, and aread/recording head (FIG. 8) having a high efficiency read back elementbased on the giant magnetoresistive (GMR) effect.

The magnetic storage apparatus is of a well-known structure and includesa magnetic recording medium (disk) 91, a driving section 92 for rotatingthe magnetic recording medium 91, a magnetic head 93 having a magneticwrite section and a read back section, a magnetic head driving section94 for relatively moving the magnetic head 93 with respect to themagnetic recording medium 91, a signal processing section 95 forprocessing signals read/written by the magnetic head 93, and aload/unload mechanism 96.

FIG. 7 is a schematic cross-sectional view illustrating the disk. Thedisk includes a substrate 27 made of NiAl, a magnetic layer 26 (26′)made of CoFe and having a thickness of 30 nm, a non-magnetic layer 25(25′) made of Ru and having a thickness of 0.7 nm, a magnetic layer 24(24′) made of CoFe and having a thickness of 30 nm, a non-magneticintermediate layer 23 (23′) made of TiCr and having a thickness of 3 nm,a perpendicular magnetic layer 22 (22′) made of CoCrPt and having athickness of 25 nm, and a protective layer 21 (21′) made of carbon andhaving a thickness of 3.5 nm. The layers 26 to 21 (26′ to 21′) aredeposited in this order on the substrate 27. The magnetic layer 26(26′), the non-magnetic layer 25 (25′) and the magnetic layer 24 (24′)correspond to a magnetic under layer.

The main pole 61 of the recording head had a track width of 0.4 μm, aread back GMR head element 69 had a track width of 0.32 μm, and thespacing between the head and the medium 66 was 15 nm. The EEPR4 systemwas used as the signal processing system, and the apparatus was operatedat an in-plane recording density of 55 Gb/in². As a result, an on-trackbit error rate of 10⁻⁸ or less was obtained, and it was confirmed thatthe apparatus can operate as a high density storage apparatus having acapacity of 35 GB per disk.

EXAMPLE 3

Another evaluation was made while using a different perpendicularmagnetic recording medium with the magnetic storage apparatusschematically illustrated in FIG. 9. FIG. 10 is a schematiccross-sectional view illustrating the perpendicular magnetic recordingmedium. The perpendicular magnetic recording medium includes a substrate101 made of glass, a Co layer 102 having a thickness of 40 nm, an Rulayer 103 having a thickness of 0.7 nm, a Co layer 104 having athickness of 40 nm, a Co-based amorphous ferromagnetic layer 105 havinga thickness of 20 nm, a non-magnetic intermediate layer 106 having athickness of 2 nm, a perpendicular magnetic layer 107 made of CoCrPtBand having a thickness of 30 nm, and a protective layer 108 made ofcarbon and having a thickness of 3 nm. The layers 102 to 108 aredeposited in this order on the substrate 101.

In the perpendicular magnetic recording medium, the portion from the Colayer 102 to the Co-based amorphous ferromagnetic layer 105 functions asa magnetic under layer. The perpendicular magnetic recording medium isdifferent from that of Example 1 in that a Co-based amorphousferromagnetic layer is used as a portion of the under layer, therebyimproving the crystallographic orientation of the perpendicular magneticlayer. In this example, a CoZrTa layer was used as the Co-basedamorphous ferromagnetic layer. It was confirmed that the recordingmedium of this example had an S/N ratio of 26 dB and a resolution of13%, indicating that the characteristics thereof are further improvedfrom those of the recording medium shown in Table 1 above.

According to the present invention, it is possible to significantlyimprove the medium S/N ratio of a perpendicular magnetic recordingmedium, whereby it is possible to realize a magnetic disk apparatuscapable of high speed and high density magnetic recording. Specifically,such a magnetic disk apparatus is capable of high density magneticrecording at a density over 50 Gb/in², thereby allowing for a reductionin the size of the apparatus and/or an increase in the capacity thereof,and thus facilitating the reduction in the cost therefor by reducing thenumber of disks.

While there has been described what are at present considered to bepreferred embodiments of the present invention, it will be understoodthat various modifications may be made thereto, and it is intended thatthe appended claims cover all such modifications as fall within the truespirit and scope of the invention.

1. A perpendicular magnetic recording medium comprising: a substrate, asoft magnetic under layer formed above the substrate and a perpendicularmagnetic layer formed above the soft magnetic under layer, wherein thesoft magnetic under layer has a 1^(st) ferromagnetic layer formed abovethe substrate, a 2^(nd) ferromagnetic layer formed above the 1^(st)ferromagnetic layer and a non-magnetic layer formed between the 1^(st)ferromagnetic layer and the 2^(nd) ferromagnetic layer, a magnetizationof a magnetic domain of the 1^(st) ferromagnetic layer and amagnetization of a magnetic domain of the 2^(nd) ferromagnetic layer areantiferromagnetically coupled to each other.
 2. A perpendicular magneticrecording medium comprising: a substrate, a soft magnetic under layerformed above the substrate and a perpendicular magnetic layer formedabove the soft magnetic under layer, wherein the soft magnetic underlayer has a 1^(st) ferromagnetic layer formed above the substrate, a2^(nd) ferromagnetic layer formed above the 1^(st) ferromagnetic layerand a non-magnetic layer formed between the 1^(st) ferromagnetic layerand the 2^(nd) ferromagnetic layer, a magnetization of a magnetic domainof the 1^(st) ferromagnetic layer and a magnetization of a magneticdomain of the 2^(nd) ferromagnetic layer are antiferromagneticallycoupled to each other and, the 2^(nd) ferromagnetic layer includes aco-based amorphous ferromagnetic layer
 3. A perpendicular magneticrecording medium according to claim 2, wherein the co-based amorphousferromagnetic layer comprises CoZnTa.
 4. A perpendicular magneticrecording medium according to claim 1, wherein a non-magneticintermediate layer is formed between the 2^(nd) ferromagnetic layer andthe perpendicular magnetic layer.
 5. A perpendicular magnetic recordingmedium according to claim 2 wherein a non-magnetic intermediate layer isformed between the co-based amorphous ferromagnetic layer and theperpendicular magnetic layer.
 6. A perpendicular magnetic recordingmedium according to claim 1, wherein a thickness of the non-magneticlayer is in the range from 0.1 nm to 1 nm.
 7. A perpendicular magneticrecording medium according to claim 2, wherein a thickness of thenon-magnetic layer is in the range from 0.1 nm to 1 nm.
 8. Aperpendicular magnetic recording medium according to claim 1, whereinthe non-magnetic layer includes Ru.
 9. A perpendicular magneticrecording medium according to claim 2, wherein the non-magnetic layerincludes Ru.
 10. A perpendicular magnetic recording medium according toclaim 6, wherein a thickness of the non-magnetic layer is in the rangefrom 0.3 nm to 1 nm.
 11. A perpendicular magnetic recording mediumaccording to claim 7, wherein a thickness of the non-magnetic layer isin the range from 0.3 nm to 1 nm.